The mono-component toner incorporating no carrier is subject to more strict requirements for performance in electrification of the toner itself than a dual component toner.
The mono-component toner is required to be sufficiently electrified between toners themselves or between the toner and a developing sleeve or an electrifying member in a developing machine. Improvement in fluidity has been proposed for improving electrification. For example, toner grains subjected to mere pulverization have an irregular shape and a smaller effective surface area involved in frictional electrification while having a large surface area, and therefore, a charge density of the toner is low. Further, magnetic flocculation occurs to degrade grain fluidity, thus adversely affecting toner electrification efficiency. Improvement in fluidity is therefore essential. For this purpose, some methods have been proposed, in which toner grains are subjected to a globularization treatment. There are disclosed thermal globularization methods in which toner surface is molten, such as a thermal globularization method disclosed in Japanese Patent O.P.I. Publication Nos. 52758/1981 and 127662/1984; a method in which surface of resin grains suspended in an air stream are molten to make them globular, disclosed in Japanese Patent O.P.I. Publication No. 134650/1983; and a method in which pulverization and globularization are simultaneously conducted at increased temperature, disclosed in Japanese Patent O.P.I. Publication NO. 616127/1986. Also known is a polymerization method in which spherical toner grains are obtained by polymerization, disclosed in Japanese Patent O.P.I. Publication Nos. 121048/1981.
It is preferable to add polyolefine to toner to improve an offset property of the toner.
However, unexpected problems are caused by the magnetic toner which contains polyolefine for improvement of the offset property and is subjected to the above-mentioned treatment. For example, in the thermal globularization method, an electrification of toner increases, but fringe images are formed due to formation of toner grains with reverse polarity, which in turn reduce the toner transfer rate. On the other hand, in the toner prepared by the polymerization method, there is a tendency where electrification of a toner does not increase as expected from a toner design.